1. Field of the Invention
This invention relates to new methods of producing wear resistant oxide coatings on zirconium metal alloy shapes which will exhibit improved wear resistance. Specifically, it deals with new processes for producing improved blue-black oxide coatings on zirconium alloy shapes for producing bearings, valves or like products which are subject to wear under unusual conditions, including corrosive conditions.
2. Description of the Prior Art
The excellent resistance of zirconium to corrosion by hot water has been known for many years and was among the factors influencing its adoption as the fuel-cladding material in boiling-water and pressurized-water nuclear power reactors. Zirconium also displays excellent corrosion-resistance in many aqueous and nonaqueous media and for this reason is enjoying increasing use in the chemical process industry. A limitation to the wider application of zirconium in this area is its relatively low resistance to abrasion and its tendency to gall, a result of its relatively low hardness of about 190 BHN (200 kg mm.sup.-2) as normally used.
In the prior art, attempts have been made to produce zirconium oxide coatings on zirconium shapes for the purpose of increasing their abrasion resistance. One of these processes is that disclosed in the Watson U.S. Pat. No. 3,615,885 which discloses a procedure for developing thick (up to 0.23 mm) oxide layers on Zircaloy 2 and Zircaloy 4. However, this procedure results in significant dimensional changes especially for parts of thickness below about 5 mm, and the oxide film produced does not exhibit especially high abrasion resistance.
Another patent which discloses a method of producing a blue-black oxide coating on zirconium alloy shapes for the purpose of increasing their abrasion resistance is U.S. Pat. No. 2,987,352. Both this patent and the above-mentioned patent produce a zirconium dioxide coating on zirconium alloys by means of air oxidation. The first mentioned patent continues the air oxidation long enough to produce a beige coating of greater thickness than the blue-black coating of the latter patent. This beige coating does not have the wear resistance of the blue-black coating and thus is not applicable to parts such as bearings, sliding fittings and valves where there are two work faces in close proximity. Since the coating wears down more quickly than the blue-black oxide coating, the close tolerances are lost and the part becomes useless.
With the blue-black oxide coatings, the thickness is considerably less and the hardness of the coating is considerably greater. This type of coating lends itself to such services as mentioned above. Although the blue-black coating is a more abrasion resistant coating than the beige coating, it is a relatively thin coating. It is, therefore, desirable to produce blue-black coatings of increased abrasion resistance without producing the beige-type coatings of the prior art.